The present invention relates to a cold cathode field emission device, a process for the production thereof and a cold cathode field emission display. More specifically, it relates to a cold cathode field emission device of which tip portion has a conical form, a process for the production thereof and a flat panel type cold cathode field emission display having the above cold cathode field emission devices arranged in a two-dimensional matrix form.
Various flat panel type displays are studied for substitutes for currently main-stream cathode ray tubes (CRT). The flat type displays include a liquid crystal display (LCD), an electroluminescence display (ELD) and a plasma display (PDP). Further, a cold cathode field emission type display which can emit electrons from a solid into vacuum without relying on thermal excitation, that is, a so-called field emission display (FED) is proposed as well, and it attracts attention from the viewpoints of brightness on a screen and low power consumption.
A cold cathode field emission type display (to be sometimes simply referred to as xe2x80x9cdisplayxe2x80x9d hereinafter) generally has a structure in which a cathode panel having electron.emitting portions so as to correspond to pixels arranged in a two-dimensional matrix form and an anode panel having a fluorescent layer which emits light when excited by colliding with electrons emitted from the electron emitting portions face each other through a vacuum layer. In each pixel on the cathode panel, generally, a plurality of electron emitting portions are formed, and further, gate electrodes are also formed for extracting electrons from the electron emitting portions. A portion having the above electron emitting portion and the above gate electrode will be referred to as an field emission device hereinafter.
For attaining a large emitted electron current at a low driving voltage in the above structure, it is required to form a top end of the electron emitting portion so as to have an acutely sharpened form, it is required to increase the density of electron emitting portions that can exist in a section corresponding to one pixel by finely forming the electron emitting portions, and it is also required to decrease the distance between the top end of the electron emitting portion and the gate electrode. For materializing these, therefore, there have been already proposed field emission devices having a variety of structures.
As one of typical examples of field emission devices used in the above conventional displays, there is known a so-called Spindt type field emission device of which the electron emitting portion is composed of a conical conductive material. FIG. 51 schematically shows the above Spindt type display. The Spindt type field emission device formed in a cathode panel CP comprises a cathode electrode 201 formed on a support 200, an insulating layer 202, a gate electrode 203 formed on the insulating layer 202, and a conical electron emitting portion 205 formed in an opening portion 204 which is provided so as to penetrate the gate electrode 203 and the insulating layer 202. A predetermined number of the electron emitting portions 205 are arranged in a two-dimensional matrix form to form one pixel. An anode panel AP has a structure in which a fluorescence layer 211 having a predetermined pattern is formed on a transparent substrate 210 and the fluorescence layer 211 is covered with an anode electrode 212.
When a voltage is applied between the electron emitting portion 205 and the gate electrode 203, electrons xe2x80x9cexe2x80x9d are extracted from the top end of the electron emitting portion 205 due to a consequently generated electric field. These electrons xe2x80x9cexe2x80x9d are attracted to the anode electrode 212 of the anode panel AP to collide with the flourescence layer 211 which is a light-emitting layer formed between the anode electrode 212 and the transparent substrate 210. As a result, the flourescence layer 211 is excited to emit light, and a desired image can be obtained. The performance of the above field emission device is basically controlled by a voltage to be applied to the gate electrode 203.
The method of producing a field emission device of the above display will be outlined with reference to FIGS. 52A, 52B, 53A and 53B hereinafter. This production method is basically a method in which the conical electron emitting portion 205 is formed by vertical vapor deposition of a metal material. That is, vaporized particles comes in perpendicularly to the opening portion 204. A shielding effect of an overhanged deposit formed in the vicinities of an opening end portion of the gate electrode 203 is utilized to gradually decrease the amount of the vaporized particles which reach a bottom portion of the opening portion 204, and the electron emitting portion 205 which is a conical deposit is formed in a self-aligned manner. For facilitating the removal of an unnecessary overhanged deposit, a peeling-off layer 206 is formed on the gate electrode 203 beforehand, and the method including the formation of the peeling-off layer will be explained below.
[Step-10]
First, the cathode electrode 201 of niobium (Nb) is formed on the support 200 which is formed of, for example, glass substrate. Then, the insulating layer 202 of SiO2 and the gate electrode 203 of an electrically conductive material are consecutively formed thereon. Then, the gate electrode 203 and the insulating layer 202 are patterned to form the opening portion 204 (see FIG. 52A).
[Step-20]
Then, as shown in FIG. 52B, aluminum is deposited on the gate electrode 203 and the insulating layer 202 by oblique vapor deposition to form the peeling-off layer 206. In this case, a sufficiently large incidence angle of vaporized particles with regard to the normal of the support 200 is selected, whereby the peeling-off layer 206 can be formed on the gate electrode 203 and the insulating layer 202 with depositing almost no aluminum on the bottom of the opening portion 204. The peeling-off layer 206 is overhanged in the form of eaves from an upper end portion of the opening portion 204, and the diameter of the opening portion 204 is substantially decreased.
[step-30]
Then, an electrically conductive material such as molybdenum (Mo) is deposited on the entire surface by vertical vapor deposition. In this case, as shown in FIG. 53A, as a conductive material layer 205A having an overhanged form grows on the peeling-off layer 206, the substantial diameter of the opening portion 204 is decreased, so that vaporized particles which serve to form a deposit on the bottom of the opening portion 204 gradually comes to be limited to vaporized particles which pass a central area of the opening portion 204. As a result, a conical deposit is formed on the bottom portion of the opening portion 204, and the conical deposit works as the electron emitting portion 205.
[Step-40]
Then, as shown in FIG. 53B, the peeling-off layer 206 is removed from the surface of the gate electrode 203 by an electrochemical process and a wet process, whereby the conductive material layer 205A above the gate electrode 203 is selectively removed.
Meanwhile, the electron emitting characteristic of the field emission device having the structure shown in FIG. 53B is greatly dependent upon a distance from an edge portion 203A of the gate electrode 203 constituting the upper end portion of the opening portion 204 to a tip portion of the electron emitting portion 205. And, the above distance is greatly dependent upon the formation accuracy of the opening portion 204, the dimensional accuracy of diameter of the opening portion 204, the thickness accuracy and coverage (step coverage) of the conductive material layer 205A formed in [Step-30] and, further, the formation accuracy of the peeling-off layer 206 which is a kind of an undercoat thereof.
For producing the display constituted of a plurality of the field emission devices having uniform properties, therefore, it is required to uniformly form the conductive material layer 205A on the entire surface of a substratum. In a general deposition apparatus, however, since conductive material particles are released from a deposition source located in one point so as to have an angle spread to some extent, the thickness and the symmetry of the coverage differ from vicinities of a central portion to circumferential areas in the substratum. Therefore, heights of the electron emitting portions are liable to vary and positions of the tip portions of the electron emitting portions are liable to deviate from the centers of the opening portions 204, so that it is difficult to control the variability of distances from the tip portions of the conical electron emitting portions 205 to the gate electrodes 203. Moreover, the above variability of the distances occurs not only among lots of products but also in one lot of the products, and it causes a non-uniformity in image display characteristic of the display, for example, brightness of an image. Further, the conductive material layer 205A is generally formed as a layer having a thickness of approximately 1 xcexcm or more, and the formation thereof by a vapor deposition method takes a time period of units of several tens of hours, which involves problems that it is difficult to improve a throughput and that a large deposition apparatus is required.
Further, it is also very difficult to form the peeling-off layer 206 uniformly on the entire surface of a substratum having a large area by an oblique vapor deposition method. It is very difficult as well to deposit the peeling-off layer 206 highly accurately such that it extends from the upper end portion of the opening portion 204 formed in the gate electrode 203 so as to form eaves. Further, the formation of the peeling-off layer 206 is liable to vary not only in a plane of the support but also among lots. Moreover, not only it is very difficult to peel off the peeling-off layer 206 over the support 200 having a large area for producing a display having a large area, but also the peeling of the peeling-off layer 206 causes contamination and causes the production yield of displays to decrease.
Further to the above, the height of the conical electron emitting portion 205 is defined mainly by the thickness of the conductive material layer 205A, and the freedom in designing the electron emitting portion 205 is low. Moreover, since it is difficult to determine an height of the electron emitting portion 205 arbitrarily as required, it is inevitably required to decrease the thickness of the insulating layer 202 when the distance from the electron emitting portion 205 to the gate electrode 203 decreases. When the thickness of the insulating layer 202 is decreased, however, it is difficult to decrease the capacitance between wiring lines (between the gate electrode 203 and the cathode electrode 201), so that there are caused problems that not only a load on an electric circuit of the display increases but also the display is downgraded in in-plane uniformity and image quality.
In the electron emitting portion 205 having the above conical form, further, the electron emitting characteristic can differ depending upon the orientation of a crystal boundary of the conductive material forming the electron emitting portion 205. In the method of producing a conventional field emission device, there is known no technique for utilizing a region having an optimum orientation in a region of a conductive material layer as the electron emitting portion 205.
It is therefore an object of the present invention to provide a cold cathode field emission device (to be sometimes referred to as xe2x80x9cfield emission devicexe2x80x9d hereinafter) and a process for the production thereof, which can overcome the above production problems in a conventional Spindt type cold cathode field emission device and enables the production of a plurality of cold cathode field emission devices having uniform and excellent electron emitting characteristics by a simple method, and a cold cathode field emission display (to be sometimes referred to as xe2x80x9cdisplayxe2x80x9d hereinafter) constituted by utilizing the above field emission devices.
The cold cathode field emission device according to a first aspect of the present invention for achieving the above object is a cold cathode field. emission device comprising;
(A) a cathode electrode formed on a support,
(B) an insulating layer formed on the support and the cathode electrode,
(C) a gate electrode formed on the insulating layer,
(D) an opening portion which penetrates through the gate electrode and the insulating layer, and
(E) an electron emitting portion which is positioned at a bottom portion of the opening portion and has a tip portion having a conical form and being composed of a crystalline conductive material,
the tip portion of the electron emitting portion having a crystal boundary nearly perpendicular to the cathode electrode.
The process for the production of a cold cathode field emission device according to the first aspect of the present invention (to be referred to as xe2x80x9cproduction process according to the first aspect of the present inventionxe2x80x9d hereinafter), is a process for the production of the cold cathode field emission device according to the first aspect of the present invention and a cold cathode field emission device according to a second aspect of the present invention to be described later. That is, the process according to the first aspect of the present invention comprises the steps of;
(a) forming a cathode electrode on a support,
(b) forming an insulating layer on the support and the cathode electrode,
(c) forming a gate electrode on the insulating layer,
(d) forming an opening portion which penetrates through at least the insulating layer and has a bottom portion where the cathode electrode is exposed,
(e) forming a conductive material layer for forming an electron emitting portion on the entire surface including the inside of the opening portion,
(f) forming a mask material layer on the conductive material layer so as to mask a region of the conductive material layer positioned in the central portion of the opening portion, and
(g) etching the conductive material layer and the mask material layer under an anisotropic etching condition where an etch rate of the conductive material layer in the direction perpendicular to the support is larger than an etch rate of the mask material layer in the direction perpendicular to the support, to form, in the opening portion, the electron emitting portion which is composed of the conductive material layer and has a tip portion having a conical form.
The above step (g) is a kind of an etchback process which deliberately utilizes an etch rate difference between the mask material layer and the conductive material layer. In the present specification, xe2x80x9cetch rate in the direction perpendicular to the supportxe2x80x9d will be simply referred to as xe2x80x9cetch ratexe2x80x9d hereinafter.
The cold cathode field emission display according to a first aspect of the present invention is a display for which the cold cathode field emission devices according to the first aspect of the present invention are applied. That is, the display according to the first aspect of the present invention comprises a plurality of pixels,
each pixel being constituted of a plurality of cold cathode field emission devices and of an anode electrode and a fluorescence layer formed on a substrate so as to face a plurality of the cold cathode field emission devices,
each cold cathode field emission device comprising;
(A) a cathode electrode formed on a support,
(B) an insulating layer formed on the support and the cathode electrode,
(C) a gate electrode formed on the insulating layer,
(D) an opening portion which penetrates through the gate electrode and the insulating layer, and
(E) an electron emitting portion which is positioned at a bottom portion of the opening portion and has a tip portion having a conical form and being composed of a crystalline conductive material,
the tip portion of the electron emitting portion having a crystal boundary nearly perpendicular to the cathode electrode.
In the cold cathode field emission device, the process for the production thereof and the cold cathode field emission display according to the first aspect of the present invention, the tip portion of the electron emitting portion has a conical form and is composed of a crystalline conductive material. The electron emitting portion may be conical as a whole, or the tip portion alone may be conical like a top-sharpened pencil. The conical form includes a conical form (bottom having a circular form) and a pyramidal form (bottom having a polygonal form). The tip portion of the electron emitting portion is a portion where a high electric field is centered, and the electron emitting portion has a dimension of the micron order, so that the tip portion is liable to suffer damage while it repeatedly emits electrons. In the first aspect of the present invention, the tip portion of the electron emitting portion is composed of a crystalline conductive material, and the direction of the crystal boundary thereof is nearly perpendicular to the cathode electrode, which means that the flow of electrons in the tip portion of the electron emitting portion does not cross the crystal boundary. Therefore, the tip portion is free from a disorder caused in crystal structure, and the electron emitting portion which emits electrons by being exposed to a high electric field is improved in durability. As a result, the field emission device and the display to which the field emission devices are incorporated can be improved so as to have a longer life.
The tip portion of the electron emitting portion can be formed from any material such as a refractory metal (for example, tungsten (W), titanium (Ti), niobium (Nb), molybdenum (Mo), tantalum (Ta) and chromium (Cr)) or any one of compounds of these (for example, nitride such as TiN and silicide such as WSi2, MoSi2, TiSi2 or TaSi2) by any method so long as the orientation of the crystal boundary is aligned nearly perpendicularly to the cathode electrode, while the tip portion is preferably formed of a tungsten layer formed by a CVD method. The CVD method has the following advantages over a vapor deposition method. The throughput can be improved to a large extent since the layer formation rate by the CVD method is remarkably high, and a layer having a uniform thickness and coverage can be relatively easily formed on the whole of a substratum having a large area since the formation of the layer by the CVD method can proceed in any points so long as the points are those which can be brought into contact with a source gas present in a layer-forming atmosphere, which differs from the vapor deposition method in which vaporized particles flies from a deposition source located in one site and are deposited. The process for forming a tungsten layer by a CVD method is well established, and tungsten is a refractory metal, so that tungsten is suitable as a material for constituting the tip portion of the electron emitting portion.
There may be formed an electrically conductive adhesive layer between the electron emitting portion and the cathode electrode. The adhesive layer can be selected from layers used as a so-called barrier metal layer in a general semiconductor process, and it may be a single layer or it may be a composite layer formed of a combination of a plurality of kinds of material. However, if it is taken into account that the electron emitting portion or a sharpened portion is formed by etching the conductive material layer or a second conductive material layer (the electron emitting portion, the sharpened portion, the conductive material layer and the second conductive material layer will be sometimes referred to as xe2x80x9cconductive material layer, etc.xe2x80x9d hereinafter) in the production process according to the first aspect and the process for the production of the field emission device according to a second aspect of the present invention to be described later, the adhesive layer is preferably selected so as to satisfy that the conductive material layer, etc., and the adhesive layer can be removed at nearly the same etch rates under the same etching condition, or that even if an etch rate R1 of the conductive material layer, etc., is higher, the etch rate R1 does not exceed five times an etch rate R2 of the adhesive layer (R2xe2x89xa6R1xe2x89xa65R2). The reason therefore is as follows. The etching of the conductive material layer, etc., proceeds to expose the adhesive surface in most part of an etched surface, a reaction product by etching of the adhesive layer may be generated in a large amount, and part of the reaction product adheres to the surface of the conductive material layer, etc., and in this case, if the above reaction product by etching has too low a vapor pressure, the reaction product itself works as an etching mask, and there is a large risk that the etching of the conductive material layer, etc., may be hampered. The simplest solution is that the same electrically conductive material is used for constituting the conductive material layer, etc., and the adhesive layer so that the etch rates of these layers can be nearly equalized. When the conductive material layer, etc., and the adhesive layer are formed from the same electrically conductive material, particularly preferably, the adhesive layer is formed by a sputtering method, and the conductive material layer, etc., are formed by a CVD method.
In the field emission device or the display according to the first aspect of the present invention, a second insulating layer may be further formed on the gate electrode and the insulating layer, and a focus electrode may be formed on the second insulating layer. The focus electrode is a member provided for preventing divergence of paths of electrons emitted from the electron emitting portion in a so-called high-voltage type display in which the potential difference between the anode electrode and the cathode electrode is the order of several thousands volts and the distance between these electrodes are relatively large. When the convergence of paths of emitted electrons is improved, an optical crosstalk among pixels is decreased, color mixing particularly in color display is prevented, and further, the pixels can be finely divided to attain a higher fineness of a display screen.
In the production process according to the first aspect of the present invention,
in the step (d), an opening portion may be formed in the insulating layer, said opening portion having a wall surface having an inclination angle OW measured from the surface of the cathode electrode as a reference, and
in the step (g), a tip portion having a conical form may be formed, said tip portion having a slant of which an inclination angle xcex8e measured from the surface of the cathode electrode as a reference satisfies a relationship of xcex8w less than xcex8e less than 90xc2x0.
The above production process enables the production of a field emission device according to a second aspect of the present invention to be described later. The step (g) is a kind of an etchback process as already described. When the wall surface of the opening portion is perpendicular to the surface of the cathode electrode, however, an etching residue of the conductive material layer may remain in a corner portion of the opening portion, and under some etching conditions, the electron emitting portion having a conical tip portion and the gate electrode may short-circuit with the etching residue. If the etchback is continued for a long period of time until the etching residue is fully removed for avoiding the above short circuit, the height of the electron emitting portion is decreased to excess at the same time, and the distance from the end portion of the gate electrode to the tip portion of the electron emitting portion increases, resulting in a decrease in the electron emission efficiency.
When the inclination angle xcex8w of the wall surface of the opening portion is defined as described above, easy incidence of etching species to the conductive material layer on the wall surface is achieved as compared with a case where the wall surface is perpendicular to the surface of the cathode electrode. Since a general etchback process uses an anisotropic etching condition under which ions as etching species come almost perpendicularly to a layer to be etched, easier incidence of the etching species is attained, which leads to a decrease in the etching time period and means that the wall surface of the opening portion comes to be exposed in a short period of time. It is therefore made possible to prevent the short circuit between the gate electrode and the electron emitting portion without decreasing the height of the electron emitting portion in the opening portion (i.e., without decreasing the electron emission efficiency).
In the most general practice, the opening portion is formed in the insulating layer by an anisotropic etching method, and in this etching method, the wall surface of the opening portion can be slanted by utilizing the effect of a depositional reaction by-product on decreasing the etch rate. When it is assumed that a silicon compound such as a silicon-oxide-containing material or a silicon-nitride-containing material is used as a material for constituting the insulating layer, fluorocarbon etching gases are used as an etching gas, and a carbon-base polymer is generated as a depositional reaction by-product. For increasing a deposition amount of the carbon-base polymer in the above etching reaction system, there can be employed measures to increase the flow rate of fluorocarbon etching gases, to decrease the flow rate of an etching gas which can serve as a source for oxygen-base chemical species which promotes the combustion of the carbon-base polymer, to decrease a mean free path of ion by increasing a gas pressure, to decrease an RF power used for exciting plasma, to increase the frequency of an RF power source used for exciting plasma to inhibit the ion-sputtering-effect-based removal of the carbon-base polymer, or to decrease the temperature of a layer being etched for decreasing the vapor pressure of the carbon-base polymer. When the deposition amount of the carbon-base polymer is too large, however, the etching no longer proceeds at a practical rate, so that the above measures should be taken to such an extent that the practical etch rate is attainable.
In the cold cathode field emission device according to the first aspect of the present invention, the opening portion penetrates through the gate electrode and the insulating layer, while the step (d) of the production process, according to the first aspect of the present invention for producing the above cold cathode field emission device, describes xe2x80x9cforming an opening portion which penetrates through xe2x80x98at leastxe2x80x99 the insulating layer and has a bottom portion where the cathode electrode is exposedxe2x80x9d. That is because in some cases, the formation of the opening portion in the gate electrode and the formation of the opening portion in the insulating layer are not necessarily required to be carried out at the same time. The above case where the formation of the opening portion in the gate electrode and the formation of the opening portion in the insulating layer are not necessarily required to be carried out at the same time refers, for example, to a case where a gate electrode having an opening portion from the beginning is formed on the insulating layer and in the opening portion, part of the insulating layer is removed to form the opening portion. The above xe2x80x9cat leastxe2x80x9d is also similarly used in this sense in the step (d) of a production process according to a second aspect of the present invention to be described later.
The production process according to the first aspect of the present invention can be largely classified to first-A to first-D aspects on the basis of variations of the step (e). That is, in the process for the production of a cold cathode field emission device according to the first-A aspect of the present invention (to be referred to as xe2x80x9cproduction process according to the first-A aspect of the present inventionxe2x80x9d hereinafter), preferably,
in the step (e), a recess is formed in the surface of the conductive material layer on the basis of a step between the upper end portion and the bottom portion of the opening portion, when the conductive material layer for forming an electron emitting portion is formed on the entire surface including the inside of the opening portion, and
in the consequent step (f), the mask material layer is formed on the entire surface of the conductive material layer and then the mask material layer is removed until a flat plane of the conductive material layer is exposed, to leave the mask material layer in the recess.
Preferably, the mask material layer remaining in the recess has a nearly flat surface. When the mask material layer which has been just formed on the entire surface of the conductive material layer has a nearly flat surface, therefore, the mask material layer can be removed by an etchback method under an anisotropic etching condition, a polishing method or a combination of these methods. When the mask material layer which has been just formed on the entire surface of the conductive material layer has no nearly flat surface, the mask material layer can be removed by a polishing method.
The mask material layer in the production process according to the first-A aspect of the present invention is composed of a material which can have an etch rate lower than the etch rate of the conductive material layer in the consequent step (g) and which can have such a fluidity at a proper stage of formation so that its surface can be flattened. The material for forming the mask material layer includes, for example, a resist material, SOG (spin on glass) and polyimide-base resins. These materials can be easily applied by a spin coating method. Otherwise, there may be used a material capable of giving a layer having a surface which can be flattened by thermal reflow, such as BPGS (boro-phospho-silicate glass).
The process for the production of a cold cathode field emission device according to each of the first-B and first-C aspects according to the present invention is a process in which the conductive material layer can have a narrower region masked by the mask material layer than in the production process according to the first-A aspect of the present invention.
That is, in the process for the production of a cold cathode field emission device according to the first-B aspect of the present invention (to be referred to as xe2x80x9cproduction process according to the first-B aspect of the present inventionxe2x80x9d hereinafter), preferably,
in the step (e), a nearly funnel-like recess having a columnar portion and a widened portion communicating with the upper end of the columnar portion is formed in the surface of the conductive material layer on the basis of a step between the upper end portion and the bottom portion of the opening portion, and
in the step (f), the mask material layer is formed on the entire surface of the conductive material layer and then the mask material layer and the conductive material layer are removed in a plane which is in parallel with the surface of the support, to leave the mask material layer in the columnar portion.
Further, in the process for the production of a cold cathode field emission device according to the first-C aspect of the present invention (to be referred to as xe2x80x9cproduction process according to the first-C aspect of the present inventionxe2x80x9d hereinafter), preferably,
in the step (e), a nearly funnel-like recess having a columnar portion and a widened portion communicating with the upper end of the columnar portion is formed in the surface of the conductive material layer on the basis of a step between the upper end portion and the bottom portion of the opening portion, and
in the step (f), the mask material layer is formed on the entire surface of the conductive material layer and then the mask material layer on the conductive material layer and in the widened portion is removed to leave the mask material layer in the columnar portion.
For forming the nearly funnel-like recess in the surface of the conductive material layer in the production process according to each of the first-B and first-C aspects of the present invention, it is sufficient to terminate the formation of the conductive material layer just before the surface (front) of conductive material layer growing nearly perpendicularly to the wall surface of the opening portion comes in contact with itself nearly in the center of the opening portion. For example, when the opening portion has the form of a circular cylinder, it is required to design that the thickness of the conductive material layer be smaller than a radius of the opening portion, whereby a columnar portion having the form of a circular cylinder is formed. The diameter of the above columnar portion is generally set in the range of approximately 5 to 30%, preferably 5 to 10%, of the diameter of the opening portion. In the production process according to each of the first-B and first-C aspects of the present invention, finally, the very small mask material layer remaining in a very narrow region (i.e., columnar portion) nearly in the central portion of the opening portion works as a mask for the etchback process, so that the tip portion of the electron emitting portion being formed comes to be more sharpened. However, the above very small mask material layer is required to have sufficient etching durability. Generally preferably, a relationship of 10R3xe2x89xa6R1 is satisfied where R3 is the etch rate of the mask material layer and R1 is the etch rate of the conductive material layer. That is, the etch rate R3 of the mask material layer is approximately {fraction (1/10)} or less of the etch rate of the conductive material layer. For example, when the conductive material layer is composed of a refractory metal such as tungsten (W), titanium (Ti), niobium (Nb), molybdenum (Mo), tantalum (Ta) and chromium (Cr) or any one of compounds of these (for example, nitrides such as TiN and silicides such as WSi2, MoSi2, TiSi2 and TaSi2), the material for the mask material layer can be selected from copper (Cu), gold (Au) or platinum (Pt), and these may be used alone or in combination.
When the mask material layer is formed on the entire surface of the conductive material layer in the production process according to each of the first-B and first-C aspects of the present invention, it is required to employ a method in which the mask material layer can enter the narrow columnar portion. An electrolytic plating method or an electroless plating method is preferred therefor. When a sputtering method or a CVD method is employed, it is particularly preferred to devise for improving a step coverage. For example, when a sputtering method is employed, desirably, so-called reflow sputtering is carried out at a layer formation temperature of approximately 300xc2x0 C. or higher, or high-pressure sputtering is carried out. When a CVD method is employed, it is preferred to use a bias ECR (electron cyclotron resonance) plasma CVD apparatus.
In the process for the production of a cold cathode field emission device according to a first-D aspect of the present invention (to be referred to as xe2x80x9cproduction process according to the first-D aspect of the present inventionxe2x80x9d hereinafter), preferably,
in the step (e), an electrically conductive adhesive layer is formed on the entire surface including the inside of the opening portion prior to formation of the conductive material layer for forming an electron emitting portion, and
in the step (g), the conductive material layer, the mask material layer and the adhesive layer are etched under an anisotropic etching condition where the etch rate of the conductive material layer and an etch rate of the adhesive layer are higher than the etch rate of the mask material layer.
It has been already described that the etch rate of the conductive material layer and the etch rate of the adhesive layer are not necessarily required to be the same and may differ to some extent in practical production, while it is preferred that the etch rate R1 of the conductive material layer for forming the electron emitting portion and the etch rate R2 of the adhesive layer satisfy a relationship of R2xe2x89xa6R1xe2x89xa65R2 in the step (g). Particularly, when the conductive material layer for forming the electron emitting portion and the adhesive layer are composed of the same electrically conductive material, the above relationship may be R2≈R1.
In the production process according to each of the first-A to first-D aspects of the present invention, it is particularly preferred to form the conductive material layer by a CVD method excellent in step coverage (step covering capability) for forming the recess in the surface of the conductive material layer on the basis of a step between the upper end portion and the bottom portion of the opening portion.
The cold cathode field emission device according to a second aspect of the present invention is a cold cathode field emission device comprising;
(A) a cathode electrode formed on a support,
(B) an insulating layer formed on the support and the cathode electrode,
(C) a gate electrode formed on the insulating layer,
(D) an opening portion which penetrates through the gate electrode and the insulating layer, and
(E) an electron emitting portion which is positioned at a bottom portion of the opening portion and has a tip portion having a conical form,
wherein a relationship of xcex8w less than xcex8e less than 90xc2x0 is satisfied where xcex8w is an inclination angle of a wall surface of the opening portion measured from the surface of the cathode electrode as a reference and xcex8e is an inclination angle of slant of the tip portion measured from the surface of the cathode electrode as a reference.
The cold cathode field emission display according to a second aspect of the present invention is a display to which the field emission devices according to the second aspect of the present invention are applied. That is, the cold cathode field emission display according to the second aspect of the present invention comprises a plurality of pixels,
each pixel being constituted of a plurality of cold cathode field emission devices and of an anode electrode and a fluorescence layer formed on a substrate so as to face a plurality of the cold cathode field emission devices,
each cold cathode field emission device comprising;
(A) a cathode electrode formed on a support,
(B) an insulating layer formed on the support and the cathode electrode,
(C) a gate electrode formed on the insulating layer,
(D) an opening portion which penetrates through the gate electrode and the insulating layer, and
(E) an electron emitting portion which is positioned at a bottom portion of the opening portion and has a tip portion having a conical form,
wherein a relationship of xcex8w less than xcex8e less than 90xc2x0 is satisfied where xcex8w is an inclination angle of a wall surface of the opening portion measured from the surface of the cathode electrode as a reference and xcex8e is an inclination angle of slant of the tip portion measured from the surface of the cathode electrode as a reference.
The inclination angle xcex8w of the wall surface of the opening portion measured from the surface of the cathode electrode as a reference is selected so as to be smaller than the inclination angle xcex8e of slant of the tip portion measured from the surface of the cathode electrode as a reference (xcex8w less than xcex8e) as described above, whereby the field emission device and the display according to the second aspect of the present invention has a structure in which a short circuit between the gate electrode and the electron emitting portion is reliably prevented while these device and display have an electron emitting portion having a sufficient height. The process for the production of the cold cathode field emission device according to the second aspect of the present invention is as already described.
The cold cathode field emission device according to a third aspect of the present invention is a cold cathode field emission device comprising;
(A) a cathode electrode formed on a support,
(B) an insulating layer formed on the support and the cathode electrode,
(C) a gate electrode formed on the insulating layer,
(D) an opening portion which penetrates through the gate electrode and the insulating layer, and
(E) an electron emitting portion which is positioned at a bottom portion of the opening portion,
the electron emitting portion comprising a base portion and a conical sharpened portion formed on the base portion.
The process for the production of a cold cathode field emission device according to a second aspect of the present invention (to be referred to as xe2x80x9cproduction process according to the second aspect of the present inventionxe2x80x9d hereinafter) is a process for the production of the field emission device according to the third aspect of the present invention. That is, the production process according to the second aspect of the present invention is a process for the production of a field emission device having an electron emitting portion which comprises a base portion and a conical sharpened portion formed on the base portion, and the process comprises the steps of;
(a) forming a cathode electrode on a support,
(b) forming an insulating layer on the support and the cathode electrode,
(c) forming a gate electrode on the insulating layer,
(d) forming an opening portion which penetrates through at least the insulating layer and has a bottom portion where the cathode electrode is exposed,
(e) filling the bottom portion of the opening portion with a base portion composed of a first conductive material layer,
(f) forming a second conductive material layer on the entire surface including a residual portion of the opening portion,
(g) forming a mask material layer on the second conductive material layer so as to mask a region of the second conductive material layer positioned in the central portion of the opening portion, and
(h) etching the second conductive material layer and the mask material layer under an anisotropic etching condition where an etch rate of the second conductive material layer in the direction perpendicular to the support is higher than an etch rate of the mask material layer in the direction perpendicular to the support, to form the sharpened portion composed of the second conductive material layer on the base portion.
The cold cathode field emission display according to a third aspect of the present invention is a display to which the cold cathode field emission devices according to the third aspect of the present invention are applied. That is, the cold cathode field emission display according to the third aspect of the present invention comprises a plurality of pixels,
each pixel being constituted of a plurality of cold cathode field emission devices and of an anode electrode and a fluorescence layer formed on a substrate so as to face a plurality of the cold cathode field emission devices,
each cold cathode field emission device comprising;
(A) a cathode electrode formed on a support,
(B) an insulating layer formed on the support and the cathode electrode,
(C) a gate electrode formed on the insulating layer,
(D) an opening portion which penetrates through the gate electrode and the insulating layer, and
(E) an electron emitting portion which is positioned at a bottom portion of the opening portion,
the electron emitting portion comprising a base portion and a conical sharpened portion formed on the base portion.
In the production process according to the second aspect of the present invention, preferably, in the step (e), the first conductive material layer is formed on the entire surface including the inside of the opening portion and then the first conductive material layer is etched to fill the bottom portion of the opening portion with the base portion. Otherwise, when it is intended to flatten an upper surface of the base portion, in the step (e), the first conductive material layer is formed on the entire surface including the inside of the opening portion, further, a planarization layer is formed on the entire surface of the first conductive material layer so as to nearly flatten the surface of the planarization layer, and the planarization layer and the first conductive material layer are etched under a condition where an etch rate of the planarization layer and an etch rate of the first conductive material layer are nearly equal, whereby the bottom portion of the opening portion can be filled with the base portion having a flat upper surface.
In the cold cathode field emission device or the cold cathode field emission display according to the third aspect of the present invention, the base portion and the sharpened portion of the electron emitting portion may be composed of different electrically conductive materials. The above constitution will be sometimes referred to as a field emission device or display according to the third-A aspect of the.present invention. For forming the above field emission device, in the production process according to the second aspect of the present invention, conductive material layers of different kinds are selected for the first conductive. material layer for forming the base portion and the second conductive material layer for forming the sharpened portion. In this case, preferably, the sharpened portion which is to exposed to a high electric field is composed of a refractory metal material, and the refractory metal material includes metals such as tungsten (W), titanium (Ti), molybdenum (Mo), niobium (Nb), tantalum (Ta) and chromium (Cr), alloys containing these metal elements, and compounds containing these metal elements (for example, nitrides such as TiN and suicides such as WSi2, MoSi2, TiSi2 and TaSi2). Particularly preferably, the sharpened portion is formed by etching a tungsten (W) layer formed by a CVD method. The base portion may be composed of a refractory metal material which is selected from the above refractory metal material and differs from the refractory metal material selected for the sharpened portion, or composed of a semiconductor material such as a polysilicon containing an impurity. Preferably, the sharpened portion of the electron emitting portion is composed of a crystalline conductive material and has a crystal boundary nearly perpendicular to the cathode electrode. For forming the above sharpened portion, the first conductive material layer for forming the base portion and the second conductive material layer for forming the sharpened portion are formed by CVD methods, and the second conductive material layer is etched to leave a portion having a crystal boundary nearly perpendicular to the cathode electrode as the sharpened portion.
In the cold cathode field emission device or the cold cathode field emission display according to the third aspect of the present invention, the base portion and the sharpened portion of the electron emitting portion may be composed of the same electrically conductive material. The above constitution will be sometimes referred to as a field emission device or display according to the third-B aspect of the present invention. For forming the above field emission device, in the production process according to the second aspect of the present invention, conductive material of the same kind is selected for the first conductive material layer for forming the base portion and the second conductive material layer for forming the sharpened portion. Preferably, the sharpened portion of the electron emitting portion is composed of a crystalline conductive material and has a crystal boundary nearly perpendicular to the cathode electrode. For forming the above sharpened portion, the first conductive material layer for forming the base portion and the second conductive material layer for forming the sharpened portion are formed by CVD methods, and the second conductive material layer is etched to leave a portion having a crystal boundary nearly perpendicular to the cathode electrode as the sharpened portion.
In the cold cathode field emission device according to the third-B aspect of the present invention, the process for the production thereof and the cold cathode field emission display according to the third aspect of the present invention, the first conductive material layer and the second conductive material layer can be formed of a metal layer of a refractory metal such as tungsten (W), titanium (Ti), molybdenum (Mo), niobium (Nb), tantalum (Ta) and chromium (Cr), an alloy layer containing any one of these metal elements, or a layer of a compound containing any one of these metal elements (for example, nitrides such as TiN and silicides such as WSi2, MoSi2, TiSi2 and TaSi2), and is formed, most preferably, of a tungsten (W) layer.
In the field emission device or the display according to the third aspect of the present invention, a relationship of xcex8w less than xcex8p less than 90xc2x0 may be satisfied where xcex8w is an inclination angle of a wall surface of the opening portion measured from the surface of the cathode electrode as a reference and xcex8p is an inclination angle of slant of the sharpened portion measured from the surface of the cathode electrode as a reference. The above constitution will be sometimes referred to as a field emission device or display according to the third-C aspect of the present invention. The above field emission device can be produced by the production process according to the second aspect of the present invention in which in the step (d), formed is the opening portion having a wall surface of an inclination angle xcex8w measured from the surface of the cathode electrode as a reference in the insulating layer, and, in the step (h), formed is the sharpened portion having a slant whose inclination angle xcex8p measured from the surface of the cathode electrode as a reference satisfies a relationship of xcex8w less than xcex8p less than 90xc2x0. The reason for the above is as already explained with regard to the production process according to the second aspect of the present invention.
The production process according to.the second aspect of the present invention can be largely classified into the second-A to second-D aspects on the basis of variations of the step (f).
That is, in the process for.the production of a cold cathode field emission device according to the second-A aspect of the present invention (to be referred to as xe2x80x9cproduction process acceding to the second-A aspect of the present inventionxe2x80x9d hereinafter), preferably,
in the step (f), a recess is formed in the surface of the second conductive material layer for forming the sharpened portion on the basis of a step between the upper end portion and the bottom portion of the opening portion when the second conductive material layer for forming the sharpened portion is formed on the entire surface including the residual portion of the opening portion, and
in the step (g), the mask material layer is formed on the entire surface of the second conductive material layer and then the mask material layer is removed until a flat plane of the second conductive material layer is exposed, to leave the mask material layer in the recess. Preferably, the mask material layer remaining in the recess has a nearly flat surface. When the mask material layer which has been just formed on the entire surface of the second conductive material layer has a nearly flat surface, therefore, the mask material layer can be removed by an etchback method under an anisotropic etching condition, a polishing method or a combination of these methods. When the mask material layer which has been just formed on the entire surface of the second conductive material layer has no nearly flat surface, the mask material layer can be removed by a polishing method. The material for constituting the mask material layer includes those described with regard to the production process according to the first-A aspect of the present invention.
The process for the production of a cold cathode field emission device according to each of the second-B and second-C aspects according to the present invention is a process in which the second conductive material layer can have a narrower region masked by the mask material layer than in the production process according to the second-A aspect.
That is, in the process for the production of a cold cathode field emission device according to the second-B aspect of the present invention (to be referred to as xe2x80x9cproduction process according to the second-B aspect of the present inventionxe2x80x9d hereinafter), preferably,
in the step (f), a nearly funnel-like recess having a columnar portion and a widened portion communicating with the upper end of the columnar portion is formed in the surface of the second conductive material layer for forming the sharpened portion on the basis of a step between the upper end portion and the bottom portion of the opening portion, and
in the step (g), the mask material layer is formed on the entire surface of the second conductive material layer and then the mask material layer and the second conductive material layer are removed in a plane parallel with the surface of the support, to leave the mask material layer in the columnar portion.
Further, in the process for the production of a cold cathode field emission device according to the second-C aspect of the present invention (to be referred to as xe2x80x9cproduction process according to the second-C aspect of the present inventionxe2x80x9d hereinafter), preferably,
in the step (f), a nearly funnel-like recess having a columnar portion and a widened portion communicating with the upper end of the columnar portion is formed in the surface of the second conductive material layer for forming the sharpened portion on the basis of a step between the upper end portion and the bottom portion of the opening portion, and
in the step (g), the mask material layer is formed on the entire surface of the second conductive material layer and then the mask material layer on the second conductive material layer and in the widened portion is removed to leave the mask material layer in the columnar portion.
In the production process according to each of the second-B and second-C aspects of the present invention, conditions necessary for forming the nearly funnel-like recess in the surface of the second conductive material layer and materials that can be used for the mask material layer are as already explained with regard to the first-B and first-C aspects of the present invention.
In the cold cathode field emission device or the cold cathode field emission display according to the third aspect of the present invention, an electrically conductive adhesive layer may be formed between the base portion and the sharpened portion. In this case, the adhesive layer may be composed of an electrically conductive material which satisfies a relationship of R2xe2x89xa6R1xe2x89xa65R2 where R1 is an etch rate of the second conductive material layer for forming the sharpened portion in the direction perpendicular to the support and R2 is an etch rate of the adhesive layer in the direction perpendicular to the support. The same electrically conductive material is preferably used for constituting the sharpened portion and the adhesive layer.
In the process for the production of a cold cathode field emission device according to the second aspect, in the step (f), an electrically conductive adhesive layer may be formed on the entire surface including the residual portion of the opening portion prior to formation of the second conductive material layer for forming the sharpened portion. As the above adhesive layer, there can be used the already described adhesive layer that can be used between the cathode electrode and the electron emitting portion. Generally preferably, a relationship of 10R3xe2x89xa6R1, is satisfied where R3 is an etch rate of the mask material layer in the direction perpendicular to the support and R1 is the etch rate of the second conductive material layer in the direction perpendicular to the support. The material for the mask material layer can be selected from copper (Cu), gold (Au) or platinum (Pt), and these may be used alone or in combination.
In the process for the production of a cold cathode field emission device according to the second-D aspect of the present invention (to be referred to as xe2x80x9cproduction process according to the second-D aspect of the present inventionxe2x80x9d hereinafter), in case where the adhesive layer is formed on the entire surface including the residual portion of the opening portion, preferably,
in the step (h), the second conductive material layer, the mask material layer and the adhesive layer are etched under an anisotropic etching condition where an etch rate of the second conductive material layer and an etch rate of the adhesive layer are higher than an etch rate of the mask material layer.
It has been already described that the etch rate of the second conductive material layer and the etch rate of the adhesive layer are not necessarily required to be the same and may differ to some extent in practical production, while it is preferred that, in the step (h), the etch rate R1 of the second conductive material layer for forming the electron emitting portion and the etch rate R2 of the adhesive layer satisfy a relationship of R2xe2x89xa6R1xe2x89xa65R2. Particularly, when the second conductive material layer for forming the sharpened portion and the adhesive layer are composed of the same electrically conductive material, the above relationship may be R2≈R1.
In the production process according to each of the second-A to second-D aspects of the present invention, it is particularly preferred to form the second conductive material layer by a CVD method excellent in step coverage (step covering capability) for forming the recess in the surface of the second conductive material layer on the basis of the step between the upper end portion and the bottom portion of the opening portion.
In the cold cathode field emission device or the cold cathode field emission display according to the third aspect of the present invention, a second insulating layer may be further formed on the insulating layer and the gate electrode, and a focus electrode may be formed on the second insulating layer.
The support for constituting the cold cathode field emission device according to any one of the aspects of the present invention may be any support so long as its surface has an insulating characteristic. It can be selected from a glass substrate, a glass substrate having a surface formed of an insulating film, a quartz substrate, a quartz substrate having a surface formed of an insulating film or a semiconductor substrate having a surface formed of an insulating film. In the display of the present invention, the substrate may be any substrate so long as its surface has an insulating characteristic. It can be selected from a glass substrate, a glass substrate having a surface formed of an insulating film, a quartz substrate, a quartz substrate having a surface formed of an insulating film or a semiconductor substrate having a surface formed of an insulating film.
The material for constituting the insulating layer can be selected from SiO2, SiN, SiON or a cured product of a glass paste, and these materials may be used alone or as a laminate of a combination thereof as required. The insulating layer can be formed by a known process such as a CVD method, a coating method, a sputtering method or a printing method.
The gate electrode, the cathode electrode and the focus electrode can be formed of a layer of a metal such as tungsten (W), niobium (Nb), tantalum (Ta), titanium (Ti), molybdenum (Mo), chromium (Cr), aluminum (Al), copper (Cu) or silver (Ag), an alloy layer containing any one of these metal elements, a compound containing any one of these metal elements (for example, nitrides such as TiN and suicides such as WSi2, MoSi2, TiSi2 or TaSi2), or a semiconductor layer of diamond. In the present invention, however, the above electrodes may be disposed when the electron emitting portion is formed by etching, and it is required to select a material which can secure etching selectivity to the conductive material layer constituting the electron emitting portion.